1. Field of the Invention
The invention relates generally to methods and apparatus for drilling and completing well bores. More specifically, the invention relates to methods and apparatus for an secondary lock for a downhole tool.
2. Background Art
In the drilling, completing, or reworking of oil wells, a great variety of downhole tools are used. For example, but not by way of limitation, it is often desirable to seal tubing or other pipe in the casing of a well, such as when it is desired to pump cement or other slurry down the tubing and force the cement or slurry around the annulus of the tubing or out into a formation. In some instances, perforations in the well in one section need to be isolated from perforations in a second section of the well. Typically, the wellbore is lined with tubular or casing to strengthen the sides of the borehole and isolate the interior of the casing from the earthen walls therearound. In order to access production fluid in a formation adjacent the wellbore, the casing is perforated, allowing the production fluid to enter the wellbore and be retrieved at the surface of the well. In other situations, there may be a need to isolate the bottom of the well from the wellhead. It then becomes necessary to seal the tubing with respect to the well casing to prevent the fluid pressure of the slurry from lifting the tubing out of the well or for otherwise isolating specific zones in which a wellbore has been placed. In other situations, there may be a need to create a pressure seal in the wellbore allowing fluid pressure to be applied to the wellbore to treat the isolated formation with pressurized fluids or solids. Downhole tools, referred to as packers and bridge plugs, are designed for the aforementioned general purposes, and are well known in the art of producing oil and gas.
Traditional packers include a sealing element having anti-extrusion rings on both upper and lower ends and a series of slips above and/or below the sealing element. Typically, a setting tool would be run with the packer to set the packer. The setting may be accomplished hydraulically due to relative movement created by the setting tool when subjected to applied pressure. This relative movement causes the slips to move up cones and extend into the surrounding tubular. At the same time, the sealing element may be compressed into sealing contact with the surrounding tubular. The set may be held by a body lock ring, which may prevent reversal of the relative movement.
Conventional bridge plugs are mechanical devices including an anchoring mechanism and compressive set resilient packoff seals. FIG. 1 shows a section view of a well 10 with a wellbore 12 having a bridge plug 15 disposed within a wellbore casing 20. The bridge plug 15 is typically attached to a setting tool and run into the hole on wire line or tubing (not shown), and then actuated with, for example, a pyrotechnic or hydraulic system. As illustrated in FIG. 1, the wellbore is sealed above and below the bridge plug so that oil migrating into the wellbore through perforations 23 will be directed to the surface of the well.
FIG. 2 is a partial cross sectional view of a typical bridge plug 50. The bridge plug 50 generally includes a body portion 80, a sealing member 52 to seal an annular area between the bridge plug 50 and the inside wall of casing (not shown) therearound and frangible slips 56, 61. The sealing member 52 is disposed between an upper retaining portion, or cone, 55 and a lower retaining portion, or cone, 60. In operation, axial forces are applied to frangible slip 56 while the body 80 and frangible slip 61 are held in a fixed position. As the slip 56 moves down in relation to the body 80 and slip 61, the sealing member 52 is actuated, and the frangible slips 56, 61 are driven up cones 55, 60. In the bridge plug of FIG. 2, the frangible slips 56, 61 are “unidirectional” and are most effective against axial forces applied to the bridge plug in a single direction. The movement of the cones and slips also axially compress and radially expand the sealing member 52, thereby forcing the sealing portion radially outward from the plug to contact the inner surface of the wellbore casing. The compressed sealing member 52 provides a fluid seal to prevent the movement of fluids across the bridge plug.
In the past, downhole tools, including compression-set packers and bridge plugs with locking features, have been used to seal against the inside of the well casing or wellbore. In such downhole tools, slips are mechanically actuated to anchor the tool to the casing wall (or to the uncased wellbore). The elastomeric sealing element may then be energized by compressing the elastomeric sealing element between upper and lower cones. A lock ring having a ratchet system is often used to prevent the cones from slipping away from the seal energizing position.
It has been found that downhole tools may leak at high pressures unless they include a means for increasing the seal energization, such as a pressure responsive self-energizing feature. Leakage occurs because even when a high setting force is used to set the downhole tool seals, once the setting force is removed, the ratchet system of the lock ring will retreat slightly before being arrested by the locking effect created when the sets of ratchet teeth mate firmly at the respective bases and apexes of each. This may cause a loosening of the seal. Downhole tools are also particularly prone to leak if fluid pressures on the packers are cycled from one direction to the other.
There have been several suggested solutions in the past to the general problem of pressure-deactivation of well packers. Each of these proposed solutions attempts to increase the seal energizing force when fluid pressure is applied, in some cases from annulus pressure above or below the packer, and in at least one case from pressure applied through the central bore of the inner mandrel. An example of one such system system is disclosed in U.S. Pat. No. 4,224,987, issued Sep. 30, 1980, to Allen. Allen discloses a well packer using a combination of an upper movable sleeve and inner mandrel movement, to increase seal element energization from annulus pressure applied from above, and a movable piston to increase seal element energization from annulus pressure applied from below. An upper shoe and sleeve are slidably retained on the inner mandrel in engagement with the seal elements, and are responsive to fluid pressure applied from above. The upper shoe and sleeve move down in response to such pressure, further compressing the packer elements. From below, annulus pressure acts upwardly on a telescoping piston, forcing it further into engagement with the packer seals. Thus, the Allen device uses movable shoes/pistons both above and below the seal elements, and requires a plurality of moving sleeves, pistons, and other parts both above and below the seal elements in order to effect the disclosed self-energizing of the seals. The Allen seal elements are actuated in such a way that the movable sleeves/pistons which effect the increased energization engage the seal elements across only a part of their diameters and may cause extrusion of the elastomeric members around them at the upper and lower extremities of the stack of seal elements. Such extrusion around the sleeves and pistons may cause uneven stresses in or even damage to the seal elements, and could lead to seal failure.
Another approach to self-energization of a well packer due to pressure applied from both above and below the packer is disclosed in U.S. Pat. No. 3,459,261, issued Aug. 5, 1969, to Cochran. The Cochran device discloses a floating sleeve on which the seal element is mounted, the floating sleeve being slidable between abutments and responsive to fluid pressure applied from above and below the packer to increase the endwise compression of the seal. Like the Allen device, the Cochran packer thus has movable sleeves both above and below the seal element. The sliding sleeve of Cochran, however, must remain free to move up and down in order to effect self-energizing. Accordingly, in the event of pressure cycling, the sleeve may become stuck or may be prevented from moving fully or properly in one direction or the other to energize the seal.
Another approach to increasing seal energization is disclosed in U.S. Pat. No. 4,423,777, issued Jan. 3, 1984, to Mullins. The Mullins patent discloses a pressure chamber within a packer with dual-acting pistons, one piston setting the slips and the other piston compressing the seal elements. In the event that the seal elements begin to loosen, for example through extrusion, the Mullins patent discloses pressuring up through the central bore of the tool to the pressure chamber therewithin, thereby forcing the upper piston further into engagement with the seal elements and increasing the energization thereof.
Accordingly, there exists a need for a bridge plug which may effectively seal a wellbore and remain effective when subjected to pressures from above or below while in use. Additionally, there exists a need to effectively self-energize a seal on a downhole tool and maintain the energization of the seal when subjected to pressures from above or below the downhole tool.